Making selections from color channels
Video: Making selections from color channelsHave you ever looked at an image in which you know you need to make a selection and just felt really discouraged because it looked so complicated that you thought there is no way I can ever make that selection? Well, before you give up, think about whether there is information in the channels in the image that will help you to make a selection and there often is. For example here, I like this image of the palm tree, but the sky is really bland. I'd like to replace the sky with something more interesting like this sunset, but that means that I'm going to have to select the tree first. I can't think of any of the traditional selection tools that would do a great job of selecting this tree with its many palm fronds, so I'm going to take a look at the Channels panel and see what's there.
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Choosing the right selection tools for the job and using those tools effectively are among the most important skills for Photoshop users. In Photoshop CS4: Selections in Depth, Jan Kabili goes deep into the subject, showing the best practices for making and working with selections. Jan walks through the manual and automatic selection methods in Photoshop and offers pointers on choosing the most effective selection methods for an image. She explains the underlying relationship between selections and alpha channels, and shows how to fine-tune selections to get the desired results. Exercise files accompany the course.
- Selecting manually with the Marquee, Lasso, and Pen tools
- Saving time with automatic selection methods like the Quick Selection tool and the Magic Wand tool
- Fine-tuning selections with Refine Edge
- Working with selections in Quick Mask mode
- Moving, modifying, and combining selections
- Saving selections as alpha channels
- Making complex selections from color channels
- Using the Background Eraser tool to replace a background
- Installing and using the legacy Extract plug-in to isolate an object from its background
Making selections from color channels
Have you ever looked at an image in which you know you need to make a selection and just felt really discouraged because it looked so complicated that you thought there is no way I can ever make that selection? Well, before you give up, think about whether there is information in the channels in the image that will help you to make a selection and there often is. For example here, I like this image of the palm tree, but the sky is really bland. I'd like to replace the sky with something more interesting like this sunset, but that means that I'm going to have to select the tree first. I can't think of any of the traditional selection tools that would do a great job of selecting this tree with its many palm fronds, so I'm going to take a look at the Channels panel and see what's there.
I will click on the Channels panel tab and here I can see the three RGB, Red, Green, Blue, grayscale images that make up the composite color image that we are seeing right now. I'm going to click on each one of the three channels. First the Red channel, then the Green channel, then the Blue channel, and as I do this I'm looking for the channel in which there is the most contrast between the dark areas and the light areas. So the Blue channel looks like a good candidate. Again, here is the green channel, maybe not as good, and the red channel.
Hands down, the Blue channel has the most contrast. And that's not always the case in every image, but it is here. So I'm going to use the information in the Blue channel as the basis for our selection. To do that I'm going to make a duplicate of the blue channel by clicking on the Blue channel and dragging it down to the Create New Channel button at the bottom of the Channels panel. That makes the Blue copy channel. With the Blue copy channel selected, I'm going to go up to the Image menu and I'm going to choose Adjustments and Levels because I'm going to apply a Levels adjustment to this channel in order to increase the contrast in the channel, so that the blacks are blacker and the white are whiter and that will make for a good selection.
Here in the Levels panel, I'm going to take the White input slider and I'll drag it to the left which will brighten the brightest pixels. Then I'll take the black input slider and I'll drag that to the right and you can really see that that's making the dark pixels darker. I could fine-tune the transition between the two by experimenting with the Gray slider. Now I don't want to go so far that I lose detail here, so I'm actually going to back off on that black slider and the white slider a little bit, and I'll click OK.
With that Blue copy channel still selected, I can paint right into that channel by selecting the Brush tool. I'm going to make sure my Foreground Color is set to Black and then I'm going to come into the image and I want to paint over all these little white dots because I want all of that to be included in the selection that I'm going to make. So I'm almost making a silhouette of the tree trunk by painting with black here. If I go too far, as I just did, I can just switch to white paint by pressing X on the keyboard. I'm going to make my brush harder by holding the Shift key and pressing the Right Bracket key and then I'll just drag down that area to paint with white over those pixels.
Then I'll switch back to black by pressing the X key on my keyboard and I'm going to fill in the trunk down here, making sure that the black is as solid black as I can get it. So now what I have is basically a Grayscale mask. You've learned in other movies that you can convert the Grayscale mask in a channel into a selection and that can be done one of several ways. In the Channels panel, I can simply go to this icon to load the channel as a selection or I could hold the Command key, the Ctrl key on a PC, as I click right on the thumbnail on the Blue copy channel.
What I have now is a selection around everything except the palm tree. So I want to invert the selection, so that the palm tree is selected. To do that I'll go to the Select menu at the top of the screen and I'll choose Inverse. Now I want to see the actual photograph of the palm tree again, so I'm going to click on the RGB Composite Channel at the top of the Channels panel. And there is my palm tree with a pretty good selection around the entire thing. What I'm going to do now is get my Move tool and use it to bring the palm tree into another image, the image of the sunset.
I will go to the Layers panel in the palm tree image and I'll make sure that the tree layer is selected. Then with the Move tool, I'll click and drag up to the sunset image tab. I'll hover there until the document window switches to the sunset image. Then I'll move down into the sunset image, when I see the gray rectangle there, I'm going to hold down the Shift key in order to register the two images and I'll release my mouse. And there is my palm tree, which I managed to select inside of the palm.psd image and drag wholesale into the sunset image.
So the next time that you feel stumped by a selection, think about looking at the Channels, to see if you find a basis there for the selection that you need.
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